1. Field of the Invention
The present invention relates generally to heat transfer devices and, more particularly, to variable conductance heat pipes.
2. Description of Related Art
The reliability of electronic components decreases significantly as a result of high temperature extremes or large temperature swings, especially in circumstances where these swings or cycles are frequent. Causes of these temperature cycles include, for example, electronic loading or environmental temperature differences.
A heat pipe is a widely used device for transferring high rates of heat flow across large distances with negligible temperature drop. It generally includes a closed pressure vessel containing a working fluid (liquid and vapor) in saturated thermal equilibrium. External heat from a heat generating source is input to an evaporator section, and heat is rejected to and dissipated by an external heat sink from a condenser section. The evaporator section and condenser section are connected by a vapor flow volume and an internal capillary wick. A working fluid, such as ammonia, evaporates in the evaporator section, and the vapor flows to the condenser section and condenses, giving up its heat of vaporization to the heat pipe wall. The working fluid then returns in liquid form to the evaporator section via capillary pumping action within the wick.
The conventional heat pipe is effective in transferring a large amount of heat where a temperature difference between two places is small, but such a heat pipe can not execute a temperature control function. A Variable Conductance Heat Pipe (VCHP) is a device which provides better temperature control, i.e., maintains a heat source at a stable temperature within a few degrees of a set point, in situations where, for example, electronics equipment can either dissipate at different power levels, or the condenser or heat sink is to varying environmental temperatures. With a VCHP, the amount of heat transferred is usually controlled by blocking part of the condenser area with a non-condensable gas. The non-condensable gas, which is stored in a gas reservoir fluidly connected to the condenser of the VCHP, displaces a controlled portion of the working fluid vapor in the condenser, rendering that portion of the condenser containing the non-condensable gas thermally inactive by blocking the interior condenser surface. Heat transfer is inhibited because the working fluid vapor must diffuse through the non-condensable gas in order to reach the condenser surface. Increasing condenser blockage effectively closes the heat pipe, reducing the area available for heat transfer. As the heat load from a heat generating source is increased, the vapor pressure of the working fluid increases causing the non-condensable gas to compress and expose more of the condenser area, resulting in a passively controlled heat transfer device.
Not only does a VCHP work to maintain a relatively constant temperature despite varying heat input from heat generating sources at the evaporator end of the VCHP, but it also is effective at maintaining the heat generating source at a relatively constant temperature where there is great variation in heat sink temperature due to varying environmental conditions.
FIG. 5 shows a typical prior art variable conductance heat pipe 400 having an evaporator end 405 and a condenser end 410. The VCHP 400 comprises a hollow envelope 420, a wick 430 a working fluid (not shown), a gas reservoir 440 containing a non-condensable gas 442, and fins 450. A heat generating source, such as an electronic device 300 is in thermal contact with the evaporator end 405 of the VCHP 400.
The sensitivity or control level of the VCHP 300 is driven by the ratio of reservoir volume to condenser volume. As shown in FIG. 5, in a typical VCHP, the gas front range 444 must swing over a relatively large distance to block or expose the entire condenser area and transfer heat to all of the fins 450. This results in the requirement of a large volume reservoir to achieve a certain desired level of control.
An improved VHCP is desired.
The present invention is a heat pipe assembly comprising a first heat pipe having a condenser and a working fluid. A reservoir contains a non-condensable gas which variably permits access of the working fluid to the condenser of the first heat pipe, depending on a pressure of the working fluid. A second heat pipe has an evaporator that is in thermal contact with the first heat pipe.